Fixing minor grammar typos (#6060)

Author: randptr

cores/esp8266/WString.h

@@ -98,7 +98,7 @@ class String {
 
         // returns true on success, false on failure (in which case, the string
         // is left unchanged).  if the argument is null or invalid, the
-        // concatenation is considered unsucessful.
+        // concatenation is considered unsuccessful.
         unsigned char concat(const String &str);
         unsigned char concat(const char *cstr);
         unsigned char concat(char c);
@@ -199,7 +199,7 @@ class String {
         unsigned char startsWith(const String &prefix, unsigned int offset) const;
         unsigned char endsWith(const String &suffix) const;
 
-        // character acccess
+        // character access
         char charAt(unsigned int index) const;
         void setCharAt(unsigned int index, char c);
         char operator [](unsigned int index) const;